1. Field of the Invention
The present invention relates to an apparatus and method for increasing the sensing accuracy of touch sensor, in particular for providing greater accuracy in the signal coordinates of the equipotential lines in the x-axis and y-axis direction produced on the (surface of the) sensor when the touch screen/board is touched at a specific location with a finger or other object or tool.
2. Description of the Related Art
With reference FIG. 7, the conventional touch sensor shown therein has a layer of conductive film B (ITO layer) coated on the surface of a transparent or translucent substrate A; a conductive material C (such as silver paste or frit) printed in a generally rectangular shape on the perimeter of the conductive film B, forming a linear pattern, and four wires W1, W2, W3 and W4 connected to the four corners C1˜C4 of the conductive material C. By applying a suitable positive voltage on two corners C2,C4 and applying OV voltage on the other two corners C1,C3, an electrical field is formed on the conductive film B. The degree of impedance (or conductivity) of the conductive material C depends on the uniformity of the electrical field on the surface of the conductive film B. For instance, if the conductive material C is highly conductive, then all electrical current will flow through the conductive material C and only a small portion will flow toward the middle surface area of the conductive film B (ITO layer), so that the contacted point on the conductive film B will be difficult to sense due to overly weak voltage. As a result, the equipotential lines S in the x-axis and y-axis direction defined by the electrical field on conductive film B surface exhibit a curve or bow shape. These curved equipotential lines S are distributed on the surface contact area of the conductive film B. When that area is touched, the voltage signals in the x,y axis direction generated are inaccurate. Thus, the cursor and the contacted point appearing on the display become positionally incorrect.
The general approach to deal with this problem was to straighten the bow of equipotential lines S in the x-axis and y-axis direction. In practice, however, the number of isolator device must be increased to counteract the curved shape of the equipotential lines S, and this considerably increases the cost. Accordingly, the present inventor tried another approach which is to replace the conductive material C with a material having low conductivity but higher impedance. Such a design, however, has some disadvantages. The voltage in the mid-region between each adjacent corner (C3,C1), or the mid-region of corners C1˜C2, or the mid-region of corners C2˜C3, or the mid-region of C3˜C4, is always higher than the voltage at the end of the four corners C1, C2, C3 and C4. Thus, the equipotential lines of the electrical field are still noted to be non-linear and curved in some certain areas, and the same positional inaccuracy occurs in the coordinate represented by the voltage signal generated upon contact of the surface of the conductive film B at certain location.
To improve upon the above drawback, U.S. Pat. No. 4,661,655 reduces the bow of the equipotential lines generated on the sensor surface by utilizing electrodes 36 which are printed on the perimeter of the conductive film 10 surface in a single row and in a discontinuous and non-overlapping manner to increase the impedance of each row of electrodes. Although increasing the impedance on the electrodes can compensate and somewhat improve part of the bow of the equipotential lines, the condition of the distribution of the bow of the equipotential lines cannot be effectively minimized. Thus, the application of this patent requires reliance on additional hardware and software for correction to achieve the object of linearity.
U.S. Pat. No. 4,797,514 essentially discloses an apparatus in which the surface of a transparent or translucent substrate 14 is printed with a conductive material 16 (i.e. impedance material) in a rectangular frame shape, the conductive material 16 being continuous; and a layer of conductive film (or impedance layer) 18 is printed on the surface of the substrate 14, the conductive material 16 being interposed under the lower surface of the conductive membrane 18, and is primarily characterized in that the inner sides of the four corners in the perimeter of the conductive membrane 18 are etched to form a plurality of insulator regions 26, and between adjacent insulator regions 26 are formed resistor electrode elements 24. As current flows through between the resistor electrode elements 24, a specific voltage gradient will be generated in these plurality of insulator areas. The effective voltage gradient can offset (or compensate) the voltage drop generated by the conductive material 16, and in turn lower the voltage generated by the conductive material 16, and the curved equipotential lines caused thereby are corrected to straight lines.
The drawback in this prior patent resides in that the conductive material 16 is continuous and thus, when the junctions at the corners is input with an appropriate voltage, the ratio of the voltage drop produced between adjacent corners is considerably large. To rely solely on the single-row of insulator region 26 and resistor electrode elements 24 to decrease the voltage is insufficient. In other words, in the four corners of the active area of the sensor, equipotential lines with large curvature will still be produced thus requiring that the active area be separated from the wiring area at a specific distance to avoid the position shift arising in response to the bow-shape equipotential lines.
In light of the foregoing, a method and an apparatus for increasing the sensing accuracy of a touch sensor has been developed.